Hope for the lost

I have heard Ephraim grieving, ‘You have disciplined me, and I was disciplined, like an untrained calf; bring me back that I may be restored, for you are the Lord my God. For after I had turned away, I relented, and after I was instructed, I struck my thigh; I was ashamed, and I was confounded, because I bore the disgrace of my youth.’ 
Is Ephraim my dear son? Is he my darling child? For as often as I speak against him, I do remember him still. Therefore my heart yearns for him; I will surely have mercy on him, declares the Lord. (Jeremiah 31:18–20)

God hears His OT people pleading for mercy and restoration. They are ashamed and aware of their sin. He then says how He thinks of them with a loving heart and a promise of mercy.

I have a relative whose birth parent remarried a Jewish woman. This person has had a great influence on my relative to the point she now thinks she is Jewish. Research says you do not need to practice the religion to be Jewish, nor do you need to be born Jewish to practice the religion. 

Huh? This makes no sense to someone reading the OT, yet it does when considering verses like these:

That is why it depends on faith, in order that the promise may rest on grace and be guaranteed to all his offspring—not only to the adherent of the law but also to the one who shares the faith of Abraham, who is the father of us all, (Romans 4:16)
Know then that it is those of faith who are the sons of Abraham. And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, “In you shall all the nations be blessed.” So then, those who are of faith are blessed along with Abraham, the man of faith. (Galatians 3:7-9)

In the mind of God, those who believe in Him like Abraham believed and lived are His children. It is not those who culturally decide to join Judaism, but those who humble themselves, are ashamed of their sin, and put their faith in the mercy of God. 

This is completely foreign to my relative. Going along with religious festivities like Passover without understanding their meaning is like celebrating Santa Claus at Christmas instead of the birth of Christ.

Yet there is hope. Should this person be convicted by the Holy Spirit and grieve their sin and plead for mercy, God is willing to call her His child and reveal to her His grace through faith. While people share their faith and speak of sin and its power, unless the Spirit does it, reasoning and excuses keep her from becoming a true Jew who embraces the wonder of God being born in a manger and becoming our Messiah.

Jesus, You encourage me by Your ability to hear the cries of a human heart, even to discipline anyone enslaved by sin. An even greater blessing is to know that You love us and are willing to have mercy on all who ask You for it. May Your Spirit be at work in the lives of all — those who are near and those who are far away.